(a) Field of the Invention
The present invention relates to an active material for a battery and a method of preparing the same, and more particularly, to an active material for a battery with excellent electrochemical characteristics and thermal stability, and a method of preparing the same.
(b) Description of the Related Art
In recent times, pursuant to reductions in size and weight of portable electronic equipment, there has been a need for developing batteries for such portable equipment with high energy density and high power density. Furthermore, it is required that such batteries be produced in a cost effective manner while being reliable and safe.
Batteries are usually classified as primary batteries that can be used only once and then disposed of, and secondary batteries that can be recharged and used repeatedly. Primary batteries include manganese batteries, alkaline batteries, mercury batteries, and silver oxide batteries. Secondary batteries include lead-acid storage batteries, nickel-metal hydride (Ni—MH) batteries, sealed nickel-cadmium batteries, lithium metal batteries, lithium ion batteries, lithium polymer batteries, and lithium-sulfur batteries.
Such batteries generate electric power through using an electrochemical reaction material (referred to hereinafter simply as the “active material”) for a positive electrode and a negative electrode. Critical factors for determining battery capacity, safety and reliability are electrochemical characteristics and thermal stability of the active material and extensive research has been made to improve such factors.
Of currently available active materials, lithium metal involves high energy density per unit volume and high electronegativity, and hence it can be well adapted for use in producing high voltage battery cells. However, lithium metal also involves safety problems when it is used alone. Accordingly, it has been widely proposed that a material capable of intercalating or deintercalating lithium metal or ions might be used for the active material.
For instance, a rechargeable lithium battery cell generates electric energy by way of oxidation and reduction reactions (called “redox”) occurring when the lithium ions are intercalated and deintercalated at the positive and negative electrodes. In a rechargeable lithium battery cell, a material having a structure where lithium ions can be reversibly intercalated and deintercalated during charge and discharge is used for the positive and negative electrodes, and an organic electrolyte or a polymer electrolyte is filled between the positive and the negative electrodes.
Lithium metal has been used as the negative active material. However, with the use of lithium metal, dendrites may be formed within the battery cell, causing short circuits and explosion of the battery cell. Therefore, a carbon-based material such as amorphous carbon or crystalline carbon has replaced metallic lithium. Particularly, boron-coated graphite (BOC), where boron is added to the carbon-based material, has recently been spotlighted as a high capacity negative active material.
It has been proposed that lithium metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<×<1) and LiMnO2, or chalcogenide compounds might be used as the positive active material for the lithium secondary battery. Manganese-based positive active materials such as LiMn2O4 and LiMnO2 can be easily synthesized at a low cost while contributing little to environmental pollution. However, such a manganese-based positive active material bears a low capacity. LiCoO2 is currently the most popular material for the positive electrodes of commercially available lithium secondary battery. This compound has high electrical conductivity, high cell voltage and excellent electrode characteristics, but it involves high production costs. Among the above positive active materials, LiNiO2 involves the lowest cost while bearing the highest discharge capacity, but it cannot be easily synthesized.
95% of the batteries that are circulated throughout the world use the LiCoO2-based positive active material, and there have been continual attempts to replace this high-cost active material with a new one. Rechargeable lithium batteries using a powder of LiCoO2 for the positive electrode exhibit relatively long shelf life and an excellent discharge profile, but there is also a need to make consistent improvements to such batteries.
In order to improve the LiCoO2-based positive active material, research has been carried out on the substitution of metal oxide for a part of the Co. The Sony corporation developed a powder of LiCo1-xAlxO2 where Al2O3 is doped at about 1 to 5 wt. % and Al is substituted for part of the Co, and applied it for use in mass production. The A&TB (Asahi & Toshiba Battery Co.) company developed a positive active material where SnO2 is doped, and Sn is substituted for part of the Co.
U.S. Pat. No. 5,292,601 disclosed LixMO2 where M is one or more elements selected from Co, Ni or Mn, and x is 0.5 to 1, as an improved active material over LiCoO2. U.S. Pat. No. 5,705,291 discloses a technique where a material selected from boron oxide, boric acid, lithium hydroxide, aluminum oxide, lithium aluminate, lithium metaborate, silicon dioxide, lithium silicate or mixtures thereof is mixed with a lithiated intercalation compound, and the mixture is baked at 400° C. or more such that the oxide content is coated onto the surface of the lithiated intercalation compound.
Japanese Patent Laid-Open No. 9-55210 discloses a positive active material prepared through coating an alkoxide of Co, Al and Mn on the lithium-nickel based oxide, and heat-treating the alkoxide-coated oxide. Japanese Patent Laid-Open No. 11-16566 discloses a lithium-based oxide material coated with metal selected from Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B or Mo, and/or an oxide thereof. Japanese Patent Laid-Open No. 11-185758 discloses a positive active material where metal oxide is coated onto lithium manganese oxide through dipping, and heat treatment is performed thereto.
As shown in FIG. 1, the currently available positive electrode is fabricated through dry-mixing an active material with a conductive agent, adding the mixture to a binder-containing solution to prepare a slurry, and coating the slurry onto a current collector and pressing it to thereby form an electrode. The conductive agent has a large surface area of 2500 m2/g or more, and hence it causes an increase in the thickness of the electrode. Furthermore, the amount of active material is decreased by the amount of a conductive agent added, and this works as a hindrance to the fabrication of a high capacity battery.
In order to solve such a problem, it has been proposed that the amount of the conductive agent and the binder should be decreased. However, such a technique cannot well serve to improve the electrochemical characteristics of the battery.
In the above description, positive active materials of lithium secondary batteries and related examples of developments were explained. Recently, in relation to the tendency to develop portable electronic equipment that is more compact and lightweight, other types of batteries have the same demands for an active material that guarantees battery performance, safety and reliability. Research and development is therefore accelerated on electrochemical properties and thermal stability of positive active materials to ensure improved performance, safety and reliability of batteries.